Microfabrication processes are used in the fabrication of structures of micrometre sizes and smaller. The earliest microfabrication processes were used for integrated circuit manufacture (or semiconductor device fabrication), however, recently these processes have been applied, for example, in microelectromechanical systems (MEMS) and subfields, such as microfluidics/lab-on-a-chip, optical MEMS, etc. The miniaturization of devices presents challenges in fabrication technologies.
While microfabrication is a collection of technologies used in manufacturing micro- and nanodevices, most microfabrication processes include thin film deposition. The purpose and material used in these thin films varies depending on the type of device. Commonly, electronic devices require thin films which are conductors (metal).
Copper is often used in semiconductor device manufacturing. As the use of copper has permeated the marketplace because of its relatively low cost and processing properties, semiconductor manufacturers continue to look for ways to improve copper deposition techniques. Several processing methods have been developed to manufacture copper interconnects as feature sizes have decreased. Each processing method may increase the likelihood of errors such as copper diffusion across boundary regions, copper crystalline structure deformation, and dewetting. Physical vapour deposition (PVD), chemical vapour deposition (CVD), atomic layer deposition (ALD), chemical mechanical polishing (CMP), electrochemical plating (ECP), electrochemical mechanical polishing (ECMP), and other methods of depositing and removing copper layers utilize mechanical, electrical, or chemical methods to manipulate the copper that forms the interconnects.
Physical vapour deposition (PVD) or sputtering has been adopted as a preferred method for depositing conductor films used in semiconductor manufacturing. This has been primarily driven by the low cost, simple sputtering approach of PVD whereby relatively pure elemental or compound materials can be deposited at relatively low substrate temperatures. However, as device length scales have decreased, the step coverage limitations of PVD have increasingly become an issue since it is inherently a line-of-sight process. Step coverage refers to the difference in deposited film thickness in different parts of a micro- or nanostructure. Ideally, there is no difference in deposited film thickness throughout the structure. This is difficult or impossible to achieve using a line-of-sight process such as PVD, which limits the total number of atoms or molecules that can be delivered into a trench or via (resulting in thinner films in trenches or vias than in the rest of the structure). Consequently, PVD is unable to deposit thin continuous films of adequate thickness to coat the sides and bottoms of high aspect ratio trenches and vias.
In addition, miniaturization of devices has led to a desire for thinner seed layers, which require greater flatness and uniformity in order to plate evenly. Copper resistivity increases sharply in films less than 10 nm in thickness, leading to uneven plating. Medium/high-density plasma and ionized PVD sources have been developed in an attempt to provide film uniformity even in the more aggressive device structures. However, these sources are still not adequate and are now of such complexity that cost and reliability have become serious concerns.
CVD processes offer improved step coverage (i.e., improved film uniformity) since CVD processes can be tailored to provide conformal films. Conformality ensures the deposited films match the shape of the underlying substrate, and the film thickness inside the feature, such as a trench or via, is uniform and equivalent to the thickness outside the feature. Unfortunately, CVD requires comparatively high deposition temperatures, suffers from high impurity concentrations, which impact film integrity, and is more expensive than PVD due to long nucleation times and poor precursor gas utilization efficiency.
Atomic layer deposition (ALD) has been proposed as an alternative method to CVD for depositing conformal, ultra-thin films at comparatively lower temperatures. ALD is similar to CVD except that the substrate is sequentially exposed to one reactant at a time, or one dose of a reactant at a time. Conceptually, it is a simple process: a first reactant is introduced to a heated substrate whereby it forms a monolayer on the surface of the substrate. Excess reactant is pumped out (e.g., evacuated). Next a second reactant is introduced and reacts with the existing monolayer to form a monolayer of a desired reaction product through a “self-limiting surface reaction”. The process is self-limiting since the deposition reaction halts once the initially adsorbed (physisorbed or chemisorbed) monolayer of the first reactant has fully reacted with the second reactant. Finally, the excess second reactant is evacuated. This sequence comprises one deposition cycle. The desired film thickness is obtained by repeating deposition cycles as necessary to reach the desired film thickness. As is apparent, the sequential nature of ALD precursor deposition, reaction and alternate purging one atomic/molecular layer at a time has the disadvantage of being slower than some other deposition techniques. However it is this cycle of building up highly uniform monolayers one at a time that allows ALD to produce films of a surface uniformity, smoothness and thinness that is impossible to achieve with other techniques. This makes ALD uniquely valuable in demanding coating applications.
In practice, ALD is complicated by painstaking process optimisation wherein: 1) at least one of the reactants sufficiently adsorbs to a monolayer and 2) surface deposition reaction can occur with adequate growth rate and film purity. If the substrate temperature needed for the deposition reaction is too high, desorption or decomposition of the first adsorbed reactant occurs, thereby preventing the layer-by-layer growth process. High substrate temperatures can also lead to mobility of the coating material which may agglomerate and ruin the film flatness or the coating material can become less uniformly dispersed or become dewetted, especially at boundary regions of a substrate structure. If the temperature is too low, the deposition reaction may be incomplete (i.e., very slow), not occur at all, or lead to poor film quality (e.g., high resistivity in the case of metals and/or high impurity content). Low temperatures may also give rise to insufficient activation of the precursor to form a monolayer, or condensation of a multilayer of precursor molecules. Since ALD processes largely rely on the thermal reactivity of precursors (i.e., reactants), selection of those that fit this temperature window becomes difficult and sometimes unattainable. Due to the strict process optimisation requirements, ALD has been typically limited to the deposition of semiconducting or insulating materials as opposed to metals. Until recently ALD of metals has been confined to the use of metal halide precursors. However, halides (e.g., Cl, F, Br) are corrosive and can create reliability issues in metal interconnects.
As a result of its low resistivity, low contact resistance, and ability to enhance device performance through the reduction of resistor-capacitor time delays, copper metallization has been adopted by many semiconductor device manufacturers for production of microelectronic chips, thin-film recording heads and packaging components.
There remains a need for precursors of copper and other metals that have sufficient volatility and thermal stability to be useful in ALD.